One can only presume that Obi-Wan really did make some sort of failed roll here. Wasn't he lecturing Anakin a while back to be more careful
with his laser sword, because, "This weapon is your life!"*

What sort of multiple failures would Obi-Wan really need to go through to drop his sword and not simply Force yank it back into his hand
immediately? First he'd need to not have holstered it safely, despite being on a rapidly moving and unstable animal that requires both hands to
hold on to the reins. Then he'd have to be so shocked by the landing of his lizard mount that he actually lets go of the weapon that
represents his entire disciplined life as a Jedi. Then he'd have to have his superhuman Jedi reflexes fail to grab the sword back before
it moves out of his grasp. Then he'd have to fail to think to use the Force to simply keep it from falling and return immediately to his hand.

Okay, make that multiple failed rolls. It's amazing he didn't also fall off the lizard.